1. Field of the Invention
The invention relates generally to spray tip assemblies for airless, high pressure spraying, and more particularly to reversible spray tip assemblies provided with a tip guard for safety.
2. Description of the Related Art
Reversible spray tip assemblies are widely used for high pressure, airless spraying of paint and other fluids. In a typical reversible spray tip assembly, a small spray nozzle is carried in a cylindrical, rotatable nozzle carrier. The nozzle carrier can be rotated 180 degrees, thereby reversing the direction of paint flow through the nozzle for cleaning nozzle obstructions. Typically the nozzle carriers are interchangeable with other nozzle carriers carrying nozzles of various diameters and capacities. Prior reversible spray tip assemblies, although successful, continue to be plagued by several problems which affect their convenience, safety and utility.
Safety for the user is a primary concern. Airless high pressure sprayers eject a very high velocity, narrow jet, which disperses and slows as it atomizes. In the area near the nozzle (within approximately one inch), where the jet is most narrow and has highest velocity, there is a risk of injection injuries to a user. In recognition of this risk, prior sprayers have included various styles of spray guards to prevent the user's body from being hit by the spray jet near the spray nozzle orifice and to warn the user of the hazard.
While such spray guards reduce the risk of injection, prior spray guards have generally suffered from a tendency to accumulate paint during spraying. Accumulated paint can then drip from the guard, creating a mess and potentially staining clothing or surfaces in the work area. In addition, accumulated paint can be splattered from the tip guard by the aerodynamic forces of the spray, causing imperfections on the painted surface. When this occurs, the user may be tempted to remove the spray guard, thereby risking injury for the sake of convenience.
Some efforts have been made to reduce the tendency for the spray guard to accumulate paint. For example, U.S. Pat. No. 4,685,621 to Scherer et al. (1987) features a tip guard having two pairs of vanes extending forward and radially outward from a base, each pair of vanes joined by a crossbar. Scherer's tip guard allows air flow through the side of the spray guard, and is somewhat successful in reducing buildup of paint on the spray guard. Nevertheless, the accumulation of paint from overspray is not completely eliminated by Scherer's design, and users may still be tempted to remove the spray guard.
Another approach to the problem is taken by Eull in his U.S. Pat. No. 4,165,836 (1979). This patent describes a safety tip guard which is coupled to the sprayer in such a way that the spray tip will not operate if the tip guard is removed. This approach improves the safety of the spray guard, but paint can still accumulate and drip. In addition, the user may be forced to stop to wipe the spray guard occasionally; if the sprayer is actuated while the user has positioned fingers inside the guard for wiping, injection injury could result.
While prior attempts to improve the spray guard have improved the situation to some degree, none of the prior guards is considered convenient, safe and trouble free.
A related problem with existing reversible tip spray tips arises from their reversible tip feature. It is a major benefit of such devices that a user can easily rotate the spraying nozzle into a reverse flow position. This enables the user to quickly remove any particles that have plugged the very small orifice in the spray tip, by injecting paint through the spray tip in the reversed flow direction, dislodging the obstruction. However, with existing reversible tip devices it is possible to accidentally rotate the spray tip out of position if the tip handle gets bumped in the course of handling or moving the spray gun. It is also possible for a user to fail to rotate the spray tip completely into position before activating the sprayer. Either of these circumstances can yield a condition where the tip is not properly aligned when fluid pressure is applied, which can result in accidents ranging in severity from minor nuisance to serious injury or damage.
Prior reversible spray tips commonly include rotation stops, so that the tip cannot be overrotated inadvertently. For example, U.S. Pat. No. 4,165,836 to Eull (1979) includes a handle with a shoulder. The shoulder has a partially rounded shape to permit tip rotation and a flattened portion which contacts a flange to limit the range of rotation. While it does prevent overrotation, the flattened portion of the shoulder does not prevent improper positioning by underrotation of the tip. Other tips similarly limit the range of rotation but do not positively lock the tip into position. Thus, prior spray tips do not completely solve the problem of inadvertent tip misalignment.
In addition to misalignment problems, prior reversible spray tips are subject to "spitting" and dripping problems when the spray gun is being triggered on or off. These problems are related to the seal design. For sealing the rotatable metal cylinder, a floating cylinder seal is commonly provided with a forward sealing face that conforms with the outer cylindrical contour. High pressure tends to force the floating seal into sealing engagement with the cylinder during spraying, preventing leakage.
To prevent leakage during start up conditions, an initial compressive loading is typically applied to the seal. For example, in the U.S. Pat. No. 4,715,537 to Calder (1987), the floating seal is biased by a spring to provide initial sealing pressure during start up. The floating seal is sealed against leakage from its rearward face by an annular (O-ring) seal.
Existing seals exhibit, in varying degrees, a tendency to cause a "spit" or drip from the spray nozzle, particularly when pressure is suddenly removed. Moreover, these seals in many cases are difficult to assemble in proper alignment, as is necessary for an effective seal. Some existing tips have a further problem: when the rotatable metal cylinder is partially rotated out of alignment with the fluid supply port, seal leakage can occur due to the paint "bridging" the seal between the port and an outside surface. This troublesome "bridging" situation is illustrated by FIG. 1. This figure shows the position of the nozzle carrier 1 when it has been turned partially so that the nozzle axis 2 does not align with the longitudinal axis 3 of the fluid passage 4. If the dimension w.sub.o is not sufficiently narrow to be fully covered by the concave face 5 of the piston seal 6 while in this intermediate position, the seal formed by the contact between the concave face 5 and the nozzle carrier 1 is bridged, and fluid (symbolized by flow line 7) is allowed to escape by flowing around the concave seal face 5. Therefore, to prevent bridging the seal, the arc defined by the opening of the rear nozzle carrier orifice 8 must be smaller than the arc defined by the concave seal face 5. This limitation is defined by a complex relationship, but for small concave faces (as used for practical sealing faces) and assuming that the fluid passage 4 is centered in the piston seal 6, it is sufficient to prevent bridging if the width w.sub.o is less than (d.sub.ps -w)/2, where w is the width of the fluid passage 4, d.sub.ps is the outside diameter of the piston seal 6, and w.sub.o is the width of the rear orifice in the spray nozzle carrier 1.
Prior reversible spray tips have had problems related to the manner of retaining a spray nozzle 9 in the rotatable cylindrical spray nozzle carrier 1. Typically, a small tungsten carbide spray nozzle is installed in a transverse bore of the nozzle carrier 1, so that the axis of the nozzle is perpendicular to the axis of the nozzle carrier 1. The transverse bore of the carrier 1 has a small step or bevel 10 which limits movement of the spray nozzle in the forward direction. A retainer 11 is installed behind the nozzle to secure its position in the bore. The nozzle must be mechanically retained in the carrier 1 such that fluid will not leak past the nozzle in either the forward or reverse flow direction. Furthermore, the nozzle must be mechanically retained in the carrier securely, to prevent it from being dislodged or ejected under very high fluid pressure (as high as 25,000 P.S.I in either the forward or reverse direction). It is also desirable that, in the reverse flow direction, some device is provided to diffuse the fluid stream to reduce the potential of injury from fluid injection while cleaning the spray tip by reverse flow. A transverse pin is often positioned across the fluid flow port for this purpose.
Previous reversible spray tips have generally retained the spray nozzles in the cylindrical carriers by either (a) threading the retainer into the carrier behind the nozzle, or (b) press fitting the retainer into the carrier behind the nozzle. The threaded retainer has high reverse load capacity but is costly and difficult to assemble. The difficulty arises because the spray pattern is not circularly symmetrical. The asymmetrical spray pattern must be oriented to the axis of the carrier (and therefore also to the spray tip assembly) to orient the maximum pattern width in the direction of spray gun movement. Since the threaded spray nozzle is rotating as it is screwed into the retainer, it is difficult to effect and maintain precise alignment of the nozzle in its seated position.
With a press fitted nozzle retainer, on the other hand, rotational alignment is not as great a problem. However, press fitting requires very tight tolerances and precise pressing technique to insure retention. In addition, the wall thickness of the retainer must be heavy enough to provide high compression pressure at the press fit interface. The wall thickness required causes the press fit hole to be so large that it will sometimes bridge the fluid seal in some positions and allow troublesome fluid leakage.
Some prior reversible spray tips have an additional problem related to the seal between the rearward end of the floating piston seal and the forward end of the spray gun. For example, Eull in his U.S. Pat. No. 4,165,836 discloses the use of a resilient sealing member interposed between the forward face of the spray gun and the rearward face of the piston seal, the sealing member having a forward end bevel which is received by a conical seat in the piston seal. This arrangement is disadvantageous in that the inside diameter of the sealing member is exposed to the fluid to be sprayed. The resilient sealing member is typically made from an organic elastomer, which can undergo chemical reactions with the fluid being sprayed, causing the elastomer to swell. The swelling of the elastomer then tends to constrict or choke off the flow of fluid through the tip, rendering the spray tip inoperable. In addition, the resilient sealing member contributes to "spitting" through the spray nozzle by reducing the rate at which fluid pressure rises and falls in response to the gun being triggered on and off.
Another problem with existing reversible tips is that they are not easily identified by the user by quick visual inspection. Although the handles of the interchangeable spray tip assemblies are frequently marked, for example with embossed part numbers, in a painting environment such markings are eventually obscured by buildup of paint or other contaminants. The paint buildup is not easily wiped from the handle, especially if it is partially dried, as is common after a long spraying session. This problem somewhat depreciates the value of the interchangeability feature of the spray tips. One cannot take full advantage of interchangeable tips if they cannot be conveniently distinguished in a workplace environment.